In television receivers with differing picture formats of, for example, 4:3 and 16:9, there exists the desire to make the picture width adjustable in the sense of a zoom effect. This means, for example, that only a fraction of 2/3 of the signal of the line scanning (trace) time fully covers the picture screen from the left edge to the right edge, i.e. the deflection velocity is increased in the area about the line center and decreased towards the start of the line and towards the end of the line. As the line transformer operates as a pure switch such an alteration of the picture width through alteration of the drive control of the output stage transistor is not possible without further measures.
The invention is based on the task to create a simple circuit for a continuous adjustment of the picture width in the sense of a zoom effect which affects the current and voltage conditions at the line output stage transistor and the high voltage as little as possible.
Thus, with the invention, in principle, a further capacitor is allocated to the tangent capacitor present anyway. This further capacitor is controlled to be conductive always from the start at the line center for an adjustable period. Therefore, the invention does not interfere with the function as such of the line output stage transistor. Rather, it is based on a conscious modification of the tangent correction or S correction through the tangent capacitor wired in series with the deflection coils. Thus, the possibility of influencing the shape of the line deflection current by means of the tangent capacitor is exploited in an advantageous manner additionally for the continuous adjustment of the picture width in the sense of a zoom effect. The zoom effect, for example, the magnification of the picture width through increasing the deflection velocity, consequently happens not by means of amplitude alteration of control signals but merely through an alteration of the tuning of the line output stage by means of a conscious alteration of the effective value of the overall effective tangent capacitor.
An essential advantage of the invention lies in the fact that a relatively large ratio between the minimum and the maximum picture width can be achieved while there ensues no adverse influence on the high voltage, the East/West pin cushion correction circuit or the line output stage transformer. An essential fact hereby is that the current through the line output stage transformer need not be increased and, therefore, this transistor is not additionally stressed or endangered. The tangent capacitor and the additional capacitor can lie in the primary circuit, i.e. immediately in series with the deflection coils. It is of advantage to dispose the tangent capacitor in the secondary circuit of an additional transformer the primary winding of which lies in series with the deflection coils. In this solution the polarity and amplitude of the effective voltages, in particular of the parabolic voltage at the tangent capacitor, can be better controlled.
In a television receiver, in particular with differing picture formats, there exists the desire to make the picture width continuously adjustable in the sense of a zoom effect. The task is to create a simple circuit for continuous adjustment of the picture width which does not materially affect the line output stage.
A further capacitor (Cs2) is allocated to the tangent capacitor (Cs1), and a switch (S) is connected in parallel to said further capacitor. The switch is always closed starting in the center of the line for an adjustable period.